Pulping process for quality protection including methods for hemicellulose extraction and treatment of hemicellulose-extracted lignocellulosic materials

ABSTRACT

A method for producing pulp by extracting hemicellulosic materials from lignocellulosic materials using water in an extraction stage, wherein the extraction stage is either a single extraction or a double extraction process; treating the lignocellulosic materials with an oxidizing agent in a treatment stage, wherein the treatment stage is selected from the group consisting of a second extraction process, an agent impregnation process, and a first pretreatment process; treating the lignocellulosic materials with a reducing agent in the treatment stage, wherein the treatment stage is selected from the group consisting of the second extraction process, the agent impregnation process, and a second pretreatment process; and then subjecting the lignocellulosic materials to a modified Kraft pulping process to produce pulp.

STATEMENT OF RELATED APPLICATIONS

This patent application claims priority on and the benefit of U.S. provisional patent application No. 61/165,995 having a filing date of 2 Apr. 2009, which is incorporated herein in its entirety by this reference.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention is related generally to the field of pulping processes preceded by hemicellulose extraction, and is related more specifically to modified pulping processes, and particularly modified Kraft pulping processes, preceded by a hemicellulose extraction process, that maintain yield and quality of pulp. This invention also is related to the field of yield and strength advantages that can be achieved by modified pulping processes compared to Kraft pulping processes not preceded by hemicellulose extraction.

2. Prior Art

Pulping is the process of converting wood or other lignocellulosic materials into separated pulp fibers for use in, for example, paper making. Common simple pulping processes, such as shown in FIG. 1, comprise (a) mechanically grinding the lignocellulosic material into fibers and (b) chemically degrading and dissolving the lignin binding the fibers together. Chemical pulping comprises the cooking of wood chips in an aqueous mixture of chemicals so as to dissolve the lignin so as to result in the separated fibers, washing the fibers, and then optionally bleaching the fibers resulting in a pulp. The wash effluent can be sent to a recycle and chemical recovery process.

The Kraft process, such as shown in FIG. 2, is a common chemical pulping process in which the lignocellulosic material (for example cellulose, hemicelluloses, and lignin) and a white liquor (sodium hydroxide, sodium sulfide, and sodium carbonate) are cooked in a digester, resulting in pulp and a black liquor (dissolved lignin, unused chemicals in the white liquor, other chemicals, and cellulose and hemicellulose degradation products). The Kraft process as well as other current chemical processes can be inefficient, result in significant waste streams, and produce chemical wastes that contain otherwise usable components that can be turned into value added products.

BRIEF SUMMARY OF THE INVENTION

The present invention, briefly stated, includes modified pulping processes preceded by hemicellulose extraction processes in which hemicellulosic materials, such as polysaccharides and other organic compounds, are extracted from the lignocellulosic materials prior to pulping. The hemicellulosic materials are extracted prior to pulping and chemicals and/or other compounds are added to the lignocellulosic materials prior to or during pulping to compensate for the loss of the hemicellulosic materials. Using the present invention, the hemicellulosic materials can be removed as value added byproducts of the pulping process, yet the addition of the chemicals and/or other compounds to the lignocellulosic materials results in a yield and quality of pulp similar to conventional pulping processes, such as Kraft pulping processes. Thus, the present invention results in more economical pulping process, in which byproducts can be extracted and sold, while still yielding a quantity and quality of pulp similar to conventional pulping processes not having such a byproduct removal and capture.

The pulp industry has recognized that it has to continue to produce chemical pulp but also must look for additional revenue streams form existing technologies. For example, in the present invention, instead of simply taking wood chips and taking out the lignin, hemicellulosic materials can be removed and rather than leaving the hemicellulosic materials in the black liquor to be disposed of, some of the hemicellulosic materials can be removed and used to produce higher value byproducts. Most are polysaccharides (sugars) that have a potentially high value can be used to make ethanol or other chemicals, for example. However, in a typical extraction process, pulling out some of the hemicellulosic materials reduces the amount of wood that can go to pulping. If a significant amount of hemicellulosic materials are removed, the pulp may have different characteristics from the prior art pulp. The present invention addresses this by providing for the extraction of hemicellulosic materials while maintaining the pulp yield and the properties of the pulp.

The present invention comprises continuous processes for protecting hemicellulose pre-extracted lignocellulosic materials, such as wood chips, against degradation in alkaline pulping processes. One aspect of the present invention is the evolution of existing chemical pulp mills into integrated forest biorefineries (IFBR) for the production of value added products. For example, using the present invention, one selectively and efficiently can pre-extract hemicelluloses before pulping while maintaining the yield and quality of the pulp. Another aspect of the present invention is that the modified pulping process can be used in place of the Kraft process to achieve yield gain at the same level of lignin removal without any reduction in the pulp properties.

Several representative processes are disclosed herein. Each process comprises a pre-extraction of hemicellulosic materials from the lignocellulosic materials, such as wood chips. The pre-extraction can be a water extraction or other types of extraction with added chemicals. After the pre-extraction, the lignocellulosic materials are pre-treated and/or treated with certain chemicals prior to the lignocellulosic materials being subjected to a modified pulping process, such as a modified Kraft process in a digester.

A first process is a double extraction process with water in the first stage followed by alkaline sodium sulfide with additive (DE or double extraction) plus a modified Kraft pulping with anthraquinone (SK or single Kraft). A second process is a single stage extraction with water (SE or single extraction) followed by a two stage modified Kraft pulping (DK or double Kraft). A third process is a single stage polysulfide pretreatment process A fourth process is a single stage polysulfide and sodium borohydride pretreatment process. A fifth process is a two stage pretreatment process with a polysulfide first pretreatment stage followed by a polysulfide/sodium borohydride/anthraquinone second pretreatment stage, along with a polysulfide recycle in the first pretreatment stage. A sixth process is a single stage polysulfide/reducing agent/anthraquinone pretreatment process for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with a polysulfide recycle. A seventh process is a single stage polysulfide/reducing agent/anthraquinone pretreatment, without a polysulfide recycle. An eighth process is a two stage pretreatment process with a polysulfide/anthraquinone first pretreatment stage followed by a polysulfide/reducing agent/anthraquinone second pretreatment stage, along with a polysulfide recycle in the first pretreatment stage. Other processes and variations also are contemplated.

According to the present invention, one manner to maintain the yield and quality of pulp after hemicellulosic materials have been extracted involves treating the extracted lignocellulosic materials with a reducing and/or oxidizing agents such as polysulfide and/or sodium borohydride, and with anthraquinone. Practically speaking, it is preferable to use polysulfide instead of sodium borohydride, or greater relative quantities of polysulfide relative to sodium borohydride, due to the relative costs of each. The added treatment stages preferably are carried out between about 120° C.-180° C. at standard pressure. At temperatures below about 120° C. the process proceeds too slowly to be economical.

These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of a prior art pulping process.

FIG. 2 is a representation of a prior art Kraft pulping process.

FIG. 3 is a block diagram of a double extraction process with water in the first stage followed by alkaline sodium sulfide with additive (DE or double extraction) plus a modified Kraft pulping with anthraquinone (SK or single Kraft).

FIG. 4 is a block diagram of a single stage extraction with water (SE or single extraction) followed by a two stage modified Kraft pulping (DK or double Kraft).

FIG. 5 is a block diagram for a single stage polysulfide pretreatment process for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials.

FIG. 6 is a block diagram for a single stage polysulfide and sodium borohydride pretreatment process for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials.

FIG. 7 is a block diagram for a two stage pretreatment process, the first stage with polysulfide and the second stage with polysulfide along with sodium borohydride as reducing agents and with anthraquinone for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with polysulfide recycle in the first pretreatment stage.

FIG. 8 is a block diagram for a single stage polysulfide pretreatment process for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with polysulfide recycle.

FIG. 9 is a block diagram for a single stage dual pretreatment of polysulfide and a reducing agent for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials without polysulfide recycle.

FIG. 10 is a block diagram for a two stage pretreatment process of polysulfide followed by a reducing agent for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with polysulfide recycle.

A table and various graphs are appended to this specification and are described in more detail in the Detailed Description of Preferred Embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to the figures, embodiments of the present invention are shown. The figures and the following description are for a limited number of embodiments for ease of understanding. However, the invention is not limited to these illustrative embodiments. Other processes and variations also are contemplated. For example, FIGS. 3-10 show block diagrams of representative Processes 1-8, respectively. Table 1 describes the conditions for various illustrative processes. Graphs 1-28 show results of the various illustrative processes and comparisons to controls such as prior art pulp and paper made from prior art pulp.

The present invention comprises continuous processes for protecting hemicellulose pre-extracted lignocellulosic materials, such as wood chips, against degradation in alkaline pulping processes. After pre-extraction, the lignocellulosic materials are treated with reducing and/or oxidizing agents before or during the alkaline pulping process to maintain the yield and quality of the pulp produced.

Several representative processes are disclosed herein. Process 1 is a double extraction process with water in the first stage followed by alkaline sodium sulfide with additive (DE or double extraction) plus a modified Kraft pulping with anthraquinone (SK or single Kraft). Process 2 is a single stage extraction with water (SE or single extraction) followed by a two stage modified Kraft pulping (DK or double Kraft). Process 3 is a single stage polysulfide pretreatment process. Process 4 is a single stage polysulfide and sodium borohydride pretreatment process. Process 5 is a two stage pretreatment process with a polysulfide first pretreatment stage followed by a polysulfide/sodium borohydride/anthraquinone second pretreatment stage, along with a polysulfide recycle in the first pretreatment stage. Process 6 is a single stage polysulfide/reducing agent/anthraquinone pretreatment process for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with a polysulfide recycle. Process 7 is a single stage polysulfide/reducing agent/anthraquinone pretreatment, without a polysulfide recycle. Process 8 is a two stage pretreatment process with a polysulfide/anthraquinone first pretreatment stage followed by a polysulfide/reducing agent/anthraquinone second pretreatment stage, along with a polysulfide recycle in the first pretreatment stage.

One manner to maintain the yield and quality of pulp after hemicellulosic materials have been extracted involves treating the extracted lignocellulosic materials with polysulfide optionally along with a reducing agent such as sodium borohydride, and with anthraquinone. Practically speaking, it is preferable to use polysulfide instead of sodium borohydride, or greater relative quantities of polysulfide relative to sodium borohydride, due to the relative costs of each. The added treatment stages preferably are carried out at between about 120° C.-180° C. at standard pressure. At temperatures below about 120° C. the process proceeds too slowly to be economical.

The various representative processes of the invention now will be disclosed in connection with the appended figures.

FIG. 3 is a block diagram of Process 1, a double extraction process with water in the first stage followed by alkaline sodium sulfide with additive (DE or double extraction) plus a modified Kraft pulping with anthraquinone (SK or single Kraft). In the first extraction stage, water is added to the lignocellulosic materials. The lignocellulosics and water mass is then treated at an elevated temperature between 120° C. and 180° C. The organics released from the chips during this operation are mostly plant polysaccharides, acetic acid and a small amount of lignin and are removed as sugar stream 1. The extracted lignocellulosic materials are added with water to a second extraction stage, along with alkaline sodium sulfide (ASS) and sodium borohydride (SBH), and treated at an elevated temperature of between 120° C. and 180 C. The double extracted lignocellulosic materials are added with water to a digester, along with sodium hydroxide and sodium sulfide to produce pulp. The black liquor released from the digester comprises, for example, dissolved lignin, sodium sulfide, sodium borohydride, sodium hydroxide, sodium sulfide, other chemicals, and cellulose and hemicellulose degradation products.

Graph 1 illustrates the double extraction weight loss produced in Process 1. This graph shows the insensitivity of the extraction relative to a control, namely, using only a single water extraction without a second extraction using sodium borohydride or another reducing agent. In this graph, the weight loss is how much hemicellulosic material was dissolved and extracted during the extraction process(es). As can be seen, with the additives, more organics are extracted than with simple water extraction. The x-axis shows the severity of the extraction stage, the H-factor, which is a combination of time and temperature of extraction.

Graph 2 illustrates the total sugar extracts in Process 1. This graph shows the amount of sugars extracted during the extraction process(es) as a function of weight loss. As above, with the additives, more sugars are extracted than with simple water extraction. Sugar extraction is of interest as sugars have value as removed byproducts.

Graph 3 illustrates the yield gain or loss for water extraction in the control typical Kraft process with a water extraction and the double extraction single Kraft process of Process 1. This graph shows that, compared to a control process comprising just a water extraction and a Kraft process, the present process provides a better yield even though hemicellulosic material is being removed in the extraction process. Specifically, the intersection of the curves with the horizontal 0 line shows the different weight loss at the same yield for the control and Process 1. This illustrates that in the general process of the present invention, hemicellulosic material can be removed without adversely affecting the yield.

As disclosed below in conjunction with Graphs 4-10, using the double extraction single Kraft process of the present invention as illustrated by Process 1, 19-24% of the wood mass (about 9-13% sugar) was extracted without a significant loss in pulp yield. With this process, other chemicals can be recovered and still have a similar yield of pulp.

FIG. 4 is a block diagram of Process 2, a single stage extraction with water (SE or single extraction) followed by a two stage modified Kraft pulping (DK or double Kraft). In the extraction stage, water is added to the lignocellulosic materials. The lignocellosics and water mass is then treated at an elevated temperature between 120° C. and 180° C. The organics released from the chips during this operation are mostly plant polysaccharides, acetic acid and a small amount of lignin and are removed as a sugar stream. The extracted lignocellulosic materials are added with water to a combined pretreatment/digester. In the pretreatment section, alkaline sodium sulfide and sodium borohydride are added to the lignocellulosic materials and the mass is treated at an elevated temperature of between 120° C. and 180° C. in the digester section, the pretreated lignocellulosic materials are digested with sodium hydroxide and sodium sulfide to produce pulp. The black liquor released from the digester comprises, for example, dissolved lignin, sodium sulfide, sodium borohydride, sodium hydroxide, sodium sulfide, other chemicals, and cellulose and hemicellulose degradation products.

Using the single extraction two stage modified Kraft process of the present invention, 14% of the wood mass (about 10% sugar) was extracted without a significant loss in pulp yield. It is expected that from 10-14% extraction (about 7-10% sugar yield) with preserved pulp yield can be achieved using Process 2. It is further expected that with 7-8% extraction sugar yield, both the yield and strength properties of the pulp can be maintained using Process 2.

The two stage modified Kraft pulping of Process 2 involves the use of strong reducing agents such as sodium borohydride, sodium dithionate, in-situ hydrogen, and others under pressure in the pretreatment stage followed by Kraft pulping with anthraquinone as an additive in the second stage. The reducing agent helps to keep remaining hemicellulose in the final pulp, retaining the pulp properties.

Graph 3 also illustrates the yield gain or loss for water extraction in the control typical Kraft process with a water extraction and the single extraction double Kraft process of Process 2. This graph shows that, compared to a control process comprising just a water extraction and a Kraft process, the present process provides a better yield even though hemicellulosic material is being removed in the extraction process. Specifically, the intersection of the curves with the horizontal 0 line shows the different weight loss at the same yield for the control and Process 2. This illustrates that in the general process of the present invention, hemicellulosic material can be removed without adversely affecting the yield.

Graph 4 illustrates the sodium borohydride pretreatment effect on single extraction double Kraft pulp yield of Process 2. Sodium borohydride is added as a reducing agent and, when added in accordance with the present invention, has been found to maintain the pulp yield, even with the extraction of hemicellulosic materials. Specifically, this graph shows the level of sodium borohydride is needed to regain the yield loss, which is the effect of sodium borohydride on the pulp yield. By adding more sodium borohydride, the yield is increased. The Kraft Control is a pure Kraft process. The three curves illustrate Process 2 conducted at different water extraction parameters—9%, 12%, and 14% water extraction.

Paper products prepared from Processes 1 and 2 were compared to control paper products. PFI beating response, handsheet density, tensile strength, bursting strength, tearing resistance, and folding endurance of pulp prepared by Processes 1 and 2 were compared to pulp prepared from a pure Kraft process and pulp prepared from a Kraft process preceded by a water extraction. Graphs 5-10 illustrate these comparisons and show that paper products prepared by Processes 1 and 2 are comparable to paper products prepared by pure Kraft process. Three different method of the processes were carried out. The parameters of the control processes and the processes of the present invention can be found in Table 1.

Graph 5 illustrates the PFI beating response of pulp prepared from Processes 1 and 2 as compared to control pulp. Graph 6 illustrates the handsheet density of paper products prepared from Processes 1 and 2 as compared to control paper products. Graph 7 illustrates the tensile strength of paper products prepared from Processes 1 and 2 as compared to control paper products. Graph 8 illustrates the bursting strength of paper products prepared from Processes 1 and 2 as compared to control paper products. Graph 9 illustrates the tearing strength of paper products prepared from Processes 1 and 2 as compared to control paper products. Graph 10 illustrates the folding endurance of paper products prepared from Processes 1 and 2 as compared to control paper products.

As can be seen, the papermaking properties of pulp prepared from Processes 1 and 2 show faster refining response, slightly higher density, comparable or higher tensile strength, comparable or higher bursting strength, comparable or higher folding endurance, and comparable tearing resistance relative to control paper. In other words, the characteristics of paper produced from the pulp produced from Processes 1 and 2 is comparable to prior art paper. The combined effect of alkaline sodium sulfide and sodium borohydride also enables Processes 1 and 2 to maintain pulp yield and paper strength with significant wood sugar extraction. Specifically, Process 1 extracts 19-24% of wood mass with a minor change in pulp and paper properties, and Process 2 extracts up to 12% wood substance without a loss in pulp yield and paper strength.

The processes of the present invention also can be carried out with additional pretreatment steps for yield preservation, several of which are disclosed herein. One pretreatment is an oxidative pretreatment with polysulfide. Another pretreatment is an oxidative-reductive dual pretreatment using polysulfide and sodium borohydride. Another pretreatment is an oxidative and oxidative-reductive two stage pretreatment using polysulfide and dual treatment using polysulfide and sodium borohydride with polysulfide recycling. FIGS. 5-7 illustrate more general embodiments of polysulfide pretreatment processes and FIGS. 8-10 illustrate more specific embodiments of polysulfide pretreatment processes according to the present invention.

FIG. 5 is a block diagram of Process 3, a polysulfide pretreatment process comprising a water extraction stage, a polysulfide pretreatment stage, and a digester stage. In the water extraction stage, which is illustrative of an extraction stage and can be replaced with other extraction methods, lignocellulosic materials in the form of wood chips are contacted with pure water and steam. Extracted hemicellulosic materials in the form of a sugar stream is removed. Steam is recycled back to the extractor. The extracted lignocellulosic materials are introduced to a pretreatment stage along with a polysulfide solution, and the extracted lignocellulosic materials are pretreated with the polysulfide solution at a temperature of between 120° C. and 140° C. for a period of up to about 2 hours. After this pretreatment stage, the lignocellulosic materials are introduced to a digester where they are treated with a white liquor to produce pulp and a black liquor. The pretreatment stage and the digester stage can be carried out in the same vessel. Process 3 was carried out at a 14% water extraction and at four different concentrations of polysulfide solution in water—6%, 10%, 15%, and 20%.

Graph 11 illustrates the polysulfide treatment effect on yield recovery for Process 3. Control 1 is a simple Kraft process. Control 2 is a simple water extraction with a Kraft process. The other four curves are for polysulfide pretreatments according to Process 3, in the concentrations of polysulfide given above. The Kappa number is used in the pulp industry to indicate how much lignin is left in the pulp. This graph shows the pulp yield compared to various Kappa numbers and indicates Process 3 has a total pulp yield comparable to, and generally in between, the two control processes.

Graph 12 illustrates the polysulfide pretreatment effect on yield recovery with a control of 44.64% total yield at Kappa 30 for Process 3. This graph uses the data from Graph 11 and shows how at a constant Kappa number the yield obtained with the various polysulfide solution concentrations. Kappa 30 is a typical Kappa number used in the pulp industry for a pulp that will be bleached.

FIG. 6 is a block diagram of Process 4, a polysulfide and sodium borohydride dual pretreatment process comprising a water extraction stage, a polysulfide and sodium borohydride pretreatment stage, and a digester stage. In the water extraction stage, which is illustrative of an extraction stage and can be replaced with other extraction methods, lignocellulosic materials in the form of wood chips are contacted with pure water and steam. Extracted hemicellulosic materials in the form of a sugar stream is removed. Steam is recycled back to the extractor. The extracted lignocellulosic materials are introduced to a pretreatment stage along with a polysulfide solution and a sodium borohydride solution, and the extracted lignocellulosic materials are pretreated with the polysulfide solution and the sodium borohydride solution at a temperature of between 120° C. and 140° C. for a period of up to about 2 hours. After this pretreatment stage, the lignocellulosic materials are introduced to a digester where they are treated with a white liquor to produce pulp and a black liquor. The pretreatment stage and the digester stage can be carried out in the same vessel. Process 4 was carried out at 14% water extraction and five different concentrations of polysulfide solution and sodium borohydride: 6% PS+0.5% SBH; 10% PS+0.5% SBH; 15% PS+0.5% SBH; 20% PS+0.5% SBH; and 15% PS+0.4% SBH.

Graph 13 illustrates the polysulfide sodium borohydride dual treatment effect on yield recovery of Process 4. As with Graph 11, Control 1 is a simple Kraft process. Control 2 is a simple water extraction with a Kraft process. The other five curves are for polysulfide pretreatments according to Process 4 at the different concentrations of polysulfide solution in water and sodium borohydride given above. This graph shows the pulp yield at various Kappa numbers and indicates Process 4 has a total pulp yield comparable to, and generally in between, the two control processes.

Graph 14 illustrates the polysulfide solution and sodium borohydride pretreatment effect on yield recovery with a control of 44.64% total yield at Kappa 30 for Process 4. This graph uses the data from Graph 13 and shows how at a constant Kappa number the yield obtained with the various polysulfide solution with 0.5% sodium borohydride addition.

FIG. 7 is a block diagram of Process 5, a two stage pretreatment process comprising a water extraction stage, a polysulfide first pretreatment stage with polysulfide recycle, a polysulfide and sodium borohydride second pretreatment stage, and a digester stage. In the water extraction stage, which is illustrative of an extraction stage and can be replaced with other extraction methods, lignocellulosic materials in the form of wood chips are contacted with pure water and steam. Extracted hemicellulosic materials in the form of a sugar stream is removed. Steam is recycled back to the extractor. The extracted lignocellulosic materials are introduced to a first pretreatment stage along with a polysulfide solution, and the extracted lignocellulosic materials are pretreated with the polysulfide solution at a temperature of between 120° C. and 140° C. for a period of up to about 2 hours. Polysulfide solution removed from the first pretreatment stage is recycled. In this example, the feed to the first pretreatment stage comprises 30% by volume fresh polysulfide solution and 70% by volume recycled polysulfide solution. The recirculation percentage can be increased by extracting more liquid from the chips. The first pretreatment stage must be carried out in a separate vessel as it has a polysulfide recycle.

After the first pretreatment stage, the lignocellulosic materials are introduced to a second pretreatment stage along with additional polysulfide solution and with white liquor (cooking liquor in the Kraft process containing sodium hydroxide and sodium sulfide, and sodium carbonate in the case of industrial system) at a temperature of about 140° C. for a period of about 1 hour. Anthraquinone is added to get a slight yield advantage. After the second pretreatment stage, the lignocellulosic materials are introduced to a digester to produce pulp and a black liquor. The second pretreatment stage and the digester stage can be carried out in the same vessel. Process 5 was carried out at 14% water extraction and six different concentrations of polysulfide solution and sodium borohydride: 15% PS and then 0% PS+0.5% SBH; 15% PS and then 2% PS+0.5% SBH; 15% PS and then 4% PS+0.5% SBH; 15% PS and then 5% PS+0.5% SBH; 15% PS and then 6% PS+0.5% SBH; and 15% PS and then 6% PS+0.4% SBH.

Graph 15 illustrates the polysulfide sodium borohydride two stage pretreatment effect on yield recovery of Process 5. As with Graphs 11 and 13, Control 1 is a simple Kraft process. Control 2 is a simple water extraction with a Kraft process. The other six curves are for polysulfide first pretreatment and polysulfide and sodium borohydride second pretreatments according to Process 5 at different concentrations of polysulfide solution in water and sodium borohydride given above. This graph shows the pulp yield compared to various Kappa numbers and indicates Process 5 has a total pulp yield comparable to, and generally in between, the two control processes.

Graph 16 illustrates the two stage pretreatment effect on the second pretreatment yield recovery with a Control 1 of 44.64% total yield and a Control 2 of 37.82% total yield at Kappa 30 for Process 5. The center line at 40.73% yield at Kappa 30 is representative of the yield obtained by a 15% polysulfide solution pretreatment with polysulfide solution recycling, namely, the forts pretreatment. This graph uses the data from Graph 15 and shows how at a constant Kappa number the yield obtained after the second pretreatment with the various polysulfide solution and sodium borohydride concentrations.

Graph 17 illustrates a 15% polysulfide solution recycling effect on the pulp yield, with 70% recycled polysulfide and 30% fresh polysulfide. This graph shows that there is no negative effect of polysulfide solution recycling, which is a positive outcome.

A novel aspect of this invention is that the hemicellulose pre-extracted lignocellulosic materials are pretreated with oxidizing agent (OA) such as alkali metal polysulfide (PS) prior to pulping (Process 3 in FIG. 5), or with oxidizing agent and reducing agent simultaneously (Process 4 in FIG. 6) or with two stage process including polysulfide impregnation followed by reducing agent treatment with or without polysulfide and anthraquinone or its derivatives in aqueous alkaline white liquor (Process 5 in FIG. 7). In this invention, the combination of oxidizing agent and reducing agent in the treatment of hemicellulose extracted lignocellulosic materials for stabilization of residual carbohydrates prior to alkaline pulping, which leads to preserve the pulp yield, is also a novel operation.

Paper products prepared from Processes 3-5 were compared to control paper products. PFI mill refining response, apparent density, breaking length, burst index, tear index, and folding endurance of pulp prepared by Processes 3-5 were compared to pulp prepared from a pure Kraft process and pulp prepared from a Kraft process preceded by a water extraction. Graphs 18-28 illustrate these comparisons and show that paper products prepared by Processes 3-5 are comparable to paper products prepared by pure Kraft processes and by Kraft processes preceded by a water extraction. Paper products from representative versions of each of Processes 3-5 were compared with the two controls: Process 3 for a 20% PS; Process 4 for a 15% PS+0.5% SBH; and Process 5 for 15% PS and then 6% PS+0.5% SBH.

Graph 18 illustrates the PFI mill refining response for Processes 3-5 relative to a control Kraft process. Graph 19 illustrates the apparent density for Processes 3-5 relative to a control Kraft process. Graph 20 illustrates in greater detail the apparent density at 400 ml and 500 ml Canadian Standard Freeness (CSF) for Processes 3-5 relative to a control Kraft process. Graph 21 illustrates the breaking length versus CSF for Processes 3-5 relative to a control Kraft process. Graph 22 illustrates in greater detail the breaking length versus CSF at 400 ml and 500 ml CSF for Processes 3-5 relative to a control Kraft process. Graph 23 illustrates the burst index for Processes 3-5 relative to a control Kraft process. Graph 24 illustrates in greater detail the burst index at 400 ml and 500 ml CSF for Processes 3-5 relative to a control Kraft process. Graph 25 illustrates the tear index versus the breaking length for Processes 3-5 relative to a control Kraft process. Graph 26 illustrates in greater detail the tear index versus the breaking length at 400 ml and 500 ml CSF for Processes 3-5 relative to a control Kraft process. Graph 27 shows the folding endurance for Processes 3-5 relative to a control Kraft process. Graph 28 illustrates in greater detail the folding numbers at 400 ml and 500 ml CSF for Processes 3-5 relative to a control Kraft process.

Graphs 18-28 show that the present process can produce a similar paper to the controls in that the papermaking properties of pulp prepared from Processes 3-5 are comparable to prior art paper, and show that the present process can produce a paper that is the same as what was produced before by the prior art, but with the added benefit that the present process pulls out other valuable products.

FIGS. 8-10 illustrate variations on Processes 3-5 involving the addition of a reducing agent and/or anthraquinone to the pretreatments.

FIG. 8 is a block diagram of Process 6, a hemicellulose extraction stage and polysulfide impregnation stage followed by alkaline pulping of lignocellulosic materials with polysulfide recycling. In the extraction stage pure water is added to the fresh lignocellulosic materials. The lignocellulosics and water mass is then treated at an elevated temperature between 120° C. and 180° C. The organics released from the chips during this operation are mostly plant polysaccharides, acetic acid and a small amount of lignin. The extract is flashed to produce preheating steam. A portion of the aqueous extract is recycled back to the extraction vessel for the purpose of raising the sugar content of the extract. In the pretreatment stage the pre-extracted lignocellulosic materials are treated with an aqueous alkaline solution containing sufficient amount of alkali metal polysulfide with or without anthraquinone (AQ) or its derivatives and with or without reducing agent (RA) at temperature between 80° C. to 180° C. The polysulfide liquor is then withdrawn to a polysulfide storage tank for recycling. In this polysulfide impregnating operation significant part of residual carbohydrates of hemicellulose extracted lignocellulosic materials are converted into carboxyl groups which are stable against alkaline peeling reaction in the subsequent alkaline cooking stage, and thereby increasing the pulp yield. With a polysulfide recycle, one must have a separate pretreatment vessel from the digester vessel.

FIG. 9 is a block diagram of Process 7, a single stage dual pretreatment using polysulfide and RA together with or without anthraquinone or its derivatives for the alkaline cooking of hemicellulose pre-extracted lignocellulosic materials. The polysulfide in this process is in the alkaline white liquor and no polysulfide recycle is involved. In the extraction stage pure water is added to the fresh lignocellulosic materials. The lignocellulosics and water mass is then treated at an elevated temperature between 120 and 180° C. The organics released from the chips during this operation are mostly plant polysaccharides, acetic acid and a small amount of lignin. The extract is flashed to produce preheating steam. A portion of the aqueous extract is recycled back to the extraction vessel for the purpose of raising the sugar content of the extract. In the pretreatment stage the pre-extracted lignocellulosic materials are treated with an aqueous alkaline solution containing sufficient amount of sodium borohydride and alkali metal polysulfide with or without anthraquinone (AQ) or its derivatives, with or without reducing agent (RA) at temperature between 80 to 180° C. In this pretreatment operation significant part of residual carbohydrates of hemicellulose extracted lignocellulosic materials are converted into carboxyl groups which are stable against alkaline peeling reaction in the subsequent alkaline cooking stage, and thereby increasing the pulp yield. Without a polysulfide recycle, the pretreatment and the digester can be in the same vessel.

FIG. 10 is a block diagram of Process 8, a two-stage pretreatment for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials. In the first pretreatment stage the hemicellulose pre-extracted lignocellulosic materials are treated with polysulfide liquor with or without anthraquinone or its derivatives at an elevated temperature between 120 and 180° C. The polysulfide liquor is then withdrawn for recycling. In the second pretreatment stage lignocellulosic materials are treated with Kraft white liquor containing polysulfide and reducing agent with or without anthraquinone and its derivatives maintaining at a temperature below 150° C. for at least 30 minutes, and subsequently and continuously alkaline cooking to delignify the lignocellulosic materials into separable fibers. In this second treatment operation significant part of residual end groups of lignocellulosic materials are further converted into alditols and carboxyl groups by reducing agent and polysulfide or anthraquinone, respectively. Pretreatment 1, with the polysulfide recycle must be in a separate vessel from pretreatment 2 and the digester, which can be in the same vessel.

A representative method of the present invention comprises the following steps:

a) in the first impregnation stage where the hemicellulose extracted lignocellulosic materials are impregnated with an aqueous polysulfide ion containing solution at an elevated temperature below 180° C., preferably between 120° C. and 180° C., and a pH of 7.0 to 13.0; whereby the aldehyde end groups of the hemicellulose extracted lignocellulosic materials are oxidized to carboxyl groups, followed by withdrawing the polysulfide ion containing solution for recycling;

b) treating the first impregnated lignocellulosic materials with a reducing agent such as lithium aluminum hydride, alkali metal borohydride, alkali metal dithionite(hydrosulfite), alkali metal amalgam, diisobutylaluminum hydride, oxalic acid, formic acid and/or the presence of hydrogen gas in a hydroxyl ion containing solution, preferably Kraft white liquor, whereby the residual aldehyde end groups of the hemicellulose extracted lignocellulosic materials are reduced to alditols or thioalditols, and subsequently continuously digesting to allow the lignocellulosic materials to separate into fibers without much mechanical action.

In either step a) or step b) or both additional additives such as anthraquinone or its derivatives and reducing agent can also be added to increase the resulting pulp yield. The method results in a pulp yield and pulp quality with a minor change in strength properties similar to that of alkaline cooking of the lignocellosic materials alone, while the cooking time is significantly reduced compared to that of traditional alkaline cooking alone or alkaline cooking of lignocellulosic materials preceded by hemicellulose extraction. In step b) only a hydroxyl ion containing solution or Kraft white liquor can be used without additional chemical additives to maintain slightly lower pulp yields than conventional alkaline pulping. All of these methods can also be used to produce quality pulps from the fresh lignocellosic materials with no pre-extraction of hemicelluloses for the purpose of obtaining higher pulp yield than conventional alkaline processes.

Using the inventive processes disclosed herein, about 5% of the lost pulp yield (total 7%) caused by hemicellulose pre-extraction could be recovered with 15-20% polysulfide treatment prior to pulping. A complete recovery (7%) can be achieved with simultaneous pre-treatment of 15% polysulfide and 0.5% sodium borohydride. Using the two stage modified Kraft process using recycled 15% polysulfide followed by 6% polysulfide and 0.4-0.5% SBH also achieved 100% yield recovery. Additionally, the continuous recycling of 15% polysulfide maintains its yield protection efficiency in the subsequent process

There are various other methods for carrying out the present invention. A representative general method for producing pulp comprises the steps of:

a) extracting organics from lignocellulosic materials in an extraction stage, wherein the extraction stage is either a single extraction or a double extraction process;

b) treating the lignocellulosic materials with an oxidizing agent in a treatment stage, wherein the treatment stage is selected from the group consisting of a second extraction process, an agent impregnation process, and a first pretreatment process;

c) treating the lignocellulosic materials with a reducing agent in the treatment stage, wherein the treatment stage is selected from the group consisting of the second extraction process, the agent impregnation process, and a second pretreatment process; and then

d) subjecting the lignocellulosic materials to a modified Kraft pulping process to produce pulp.

The method can comprise a double extraction process with water in a first extraction stage and with the oxidizing agent along with an additive in a second extraction stage or in the first pretreatment process, followed by the Kraft pulping process with anthraquinone in a digester. The lignocellulosic materials and the water can be treated at a temperature between about 120° C. and 180° C. in the extraction stage. The extracted lignocellulosic materials can be added with water to the second extraction stage, along with the oxidizing agent and the reducing agent, and treated at an elevated temperature of between about 120° C. and 180° C. The double extracted lignocellulosic materials can be added with water to the digester, along with sodium hydroxide and sodium sulfide to produce the pulp.

The oxidizing agent can be selected from the group consisting of alkaline sodium sulfide and polysulfides and the reducing agent is selected from the group consisting of sodium borohydride, sodium dithionate, and in-situ hydrogen.

The method alternatively can comprise a single stage extraction process with water in the extraction stage, followed by a two stage modified Kraft pulping comprising a pretreatment with the oxidizing agent. The lignocellulosic materials and the water can be treated at a temperature between about 120° C. and 180° C. in the extraction stage. The extracted lignocellulosic materials can be added with additional water to the treatment stage, along with the oxidizing agent and the reducing agent, and treated at an elevated temperature of between about 120° C. and 180° C. The lignocellulosic materials can be added to the digester, along with sodium hydroxide and sodium sulfide to produce the pulp.

The method alternatively can be a single stage oxidizing agent pretreatment process for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with a polysulfide recycle further comprising the steps of:

a) impregnating hemicellulose extracted lignocellulosic materials with the oxidizing agent, wherein the oxidizing agent is an aqueous polysulfide ion containing solution, whereby aldehyde end groups of the hemicellulose extracted lignocellulosic materials are oxidized to carboxyl groups; and

b) treating the impregnated lignocellulosic materials with a reducing agent, whereby the aldehyde end groups of the hemicellulose extracted lignocellulosic materials are reduced to alditols or thioalditols, and subsequently continuously digesting the lignocellulosic materials to allow the lignocellulosic materials to separate into fibers without much mechanical action.

The method alternatively can be a single stage dual pretreatment with an oxidizing agent and a reducing agent for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials without a polysulfide recycle further comprising the steps of:

a) impregnating hemicellulose extracted lignocellulosic materials with the oxidizing agent, wherein the oxidizing agent is an aqueous polysulfide ion containing solution, whereby aldehyde end groups of the hemicellulose extracted lignocellulosic materials are oxidized to carboxyl groups; and

b) treating the impregnated lignocellulosic materials with a reducing agent, whereby the aldehyde end groups of the hemicellulose extracted lignocellulosic materials are reduced to alditols or thioalditols, and subsequently continuously digesting the lignocellulosic materials to allow the lignocellulosic materials to separate into fibers without much mechanical action.

The method alternatively can be a two stage pretreatment process with an oxidizing agent followed by a reducing agent for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with a polysulfide recycle further comprising the steps of:

a) impregnating hemicellulose extracted lignocellulosic materials with the oxidizing agent, wherein the oxidizing agent is an aqueous polysulfide ion containing solution, whereby aldehyde end groups of the hemicellulose extracted lignocellulosic materials are oxidized to carboxyl groups; and

b) treating the impregnated lignocellulosic materials with a reducing agent, whereby the aldehyde end groups of the hemicellulose extracted lignocellulosic materials are reduced to alditols or thioalditols, and subsequently continuously digesting the lignocellulosic materials to allow the lignocellulosic materials to separate into fibers without much mechanical action.

The method can be conducted at a temperature below 180° C., step a) can be conducted at a temperature between 120° C. and 180° C. and at a pH of 7.0 to 13.0, and step b) can be conducted at a temperature between 80° C. to 180° C. Additional additives can be added in either step a) or step b) or both to increase the resulting pulp yield. The additional additives can be selected from the group consisting of such as anthraquinone, anthraquinone derivatives, and reducing agents. The reducing agent is selected from the group consisting of lithium aluminum hydride, alkali metal borohydride, alkali metal dithionite(hydrosulfite), alkali metal amalgam, diisobutylaluminum hydride, oxalic acid, formic acid, and/or the presence of hydrogen gas in a hydroxyl ion containing solution such as Kraft white liquor. A portion of the extract from the extraction stage can be recycled back to the extraction stage for raising the sugar content of the extract. Although preferred embodiments include a water and/or steam extraction process, the extraction process is not limited to water or steam. In the second pretreatment process the lignocellulosic materials can be treated with Kraft white liquor containing polysulfide and reducing agent with or without anthraquinone and its derivatives, and the second pretreatment process can be maintained at a temperature below 150° C. for at least 30 minutes.

The above detailed description of the preferred embodiments, examples, and the appended figures are for illustrative purposes only and are not intended to limit the scope and spirit of the invention, and its equivalents, as defined by the appended claims. One skilled in the art will recognize that many variations can be made to the invention disclosed in this specification without departing from the scope and spirit of the invention. 

1. A method of protecting hemicellulose pre-extracted lignocellulosic materials against degradation in an alkaline pulping process comprising: a) extracting organics from lignocellulosic materials using water or steam; b) treating the lignocellulosic materials with an oxidizing agent and a reducing agent or additive, wherein the treating further includes a second extraction; and c) subjecting the lignocellulosic materials to a Kraft pulping process in a digester.
 2. The method as claimed in claim 1, wherein the extracting further comprises treating the lignocellulosic materials and the water at a temperature between about 120° C. and 180° C.
 3. The method as claimed in claim 2, wherein the extracted lignocellulosic materials are added with water to the second extraction, along with the oxidizing agent and the reducing agent, and treated at an elevated temperature of between about 120° C. and 180° C.
 4. The method as claimed in claim 3, wherein the double extracted lignocellulosic materials are added with water to the digester, along with sodium hydroxide and sodium sulfide to produce the pulp.
 5. The method as claimed in claim 4, wherein the oxidizing agent is selected from the group consisting of polysulfides and the reducing agent is selected from the group consisting of sodium borohydride, sodium dithionate, and in-situ hydrogen.
 6. A method of protecting hemicellulose pre-extracted lignocellulosic materials against degradation in alkaline pulping processes comprising: a) extracting organics from lignocellulosic materials using water or steam; b) treating the lignocellulosic materials with an oxidizing agent and a reducing agent; and c) subjecting the lignocellulosic materials to a Kraft pulping process to produce pulp in a digester.
 7. The method as claimed in claim 6, wherein the extracting further comprises treating the lignocellulosic materials and the water at a temperature between about 120° C. and 180° C.
 8. The method as claimed in claim 7, wherein the treating further comprises adding the lignocellulosic materials with additional water, along with the oxidizing agent and the reducing agent, and further treating at an elevated temperature of between about 120° C. and 180° C.
 9. The method as claimed in claim 8, wherein the treated lignocellulosic materials are added to the digester, along with sodium hydroxide and sodium sulfide to produce the pulp.
 10. The method as claimed in claim 9, wherein the oxidizing agent is selected from the group consisting of polysulfides and the reducing agent is selected from the group consisting of sodium borohydride, sodium dithionate, and in-situ hydrogen.
 11. A method of protecting hemicellulose pre-extracted lignocellulosic materials against degradation in alkaline pulping processes comprising: a) extracting organics from lignocellulosic materials using water or steam; b) impregnating the hemicellulose extracted lignocellulosic materials with an oxidizing agent, wherein the oxidizing agent is an aqueous polysulfide ion containing solution; c) treating the impregnated lignocellulosic materials with a reducing agent; and d) subjecting the lignocellulosic materials to a Kraft pulping process to produce pulp in a digester to allow the lignocellulosic materials to separate into fibers.
 12. The method as claimed in claim 11, wherein the impregnating is conducted at a temperature below 180° C.
 13. The method as claimed in claim 11, wherein the impregnating is conducted at a pH of 7.0 to 13.0.
 14. The method as claimed in claim 11, wherein in either the impregnating or the treating, further comprising adding additives to increase the resulting pulp yield.
 16. The method as claimed in claim 14, wherein the additives are selected from the group consisting of: anthraquinone, anthraquinone derivatives, and reducing agents.
 17. The method as claimed in claim 11, wherein the reducing agent is selected from the group consisting of lithium aluminum hydride, alkali metal borohydride, alkali metal dithionite(hydrosulfite), alkali metal amalgam, diisobutylaluminum hydride, oxalic acid, formic acid, and/or the presence of hydrogen gas in a hydroxyl ion containing solution such as Kraft white liquor.
 18. The method as claimed in claim 11, wherein a portion of aqueous extract from the extracting is recycled back for raising the sugar content of the extract.
 19. The method as claimed in claim 11, wherein the treating is conducted at a temperature between 80° C. to 180° C.
 20. The method as claimed in claim 11, wherein the polysulfide ion containing solution is withdrawn from the impregnating to a polysulfide storage tank for recycling.
 21. A method of protecting hemicellulose pre-extracted lignocellulosic materials against degradation in alkaline pulping processes comprising: a) extracting organics from lignocellulosic materials using water or steam; b) treating the hemicellulose extracted lignocellulosic materials with a first oxidizing agent as a first pretreatment, wherein the oxidizing agent is an aqueous polysulfide ion containing solution; c) treating the lignocellulosic materials with a second oxidizing agent and a reducing agent as a second pretreatment; and d) subjecting the lignocellulosic materials to a Kraft pulping process to produce pulp in a digester.
 22. The method as claimed in claim 21, wherein the first pretreatment is conducted at a temperature below 180° C.
 23. The method as claimed in claim 21, wherein the first pretreatment is conducted at a pH of 7.0 to 13.0.
 24. The method as claimed in claim 21, wherein in either the first pretreatment or the second pretreatment, further comprising adding additives to increase the resulting pulp yield.
 25. The method as claimed in claim 24, wherein the additives are selected from the group consisting of: anthraquinone, anthraquinone derivatives, and reducing agents.
 26. The method as claimed in claim 21, wherein the reducing agent is selected from the group consisting of lithium aluminum hydride, alkali metal borohydride, alkali metal dithionite(hydrosulfite), alkali metal amalgam, diisobutylaluminum hydride, oxalic acid, formic acid, and/or the presence of hydrogen gas in a hydroxyl ion containing solution such as Kraft white liquor.
 27. The method as claimed in claim 21, wherein a portion of aqueous extract from the extracting is recycled back for raising the sugar content of the extract.
 28. The method as claimed in claim 21, wherein the polysulfide ion containing solution is withdrawn from the first pretreatment to a polysulfide storage tank for recycling.
 29. The method as claimed in claim 21, wherein the first pretreatment is conducted at a temperature between 80° C. to 180° C.
 30. The method as claimed in claim 21, wherein in the second pretreatment the lignocellulosic materials are treated with Kraft white liquor containing polysulfide and reducing agent.
 31. The method as claimed in claim 30, wherein the second pretreatment is conducted at a temperature below 150° C. for at least 30 minutes.
 32. A system for protecting hemicellulose pre-extracted lignocellulosic materials against degradation in alkaline pulping processes, the system comprising: a) means for water extraction, wherein organics from the lignocellulosic materials are released; b) means for treatment or impregnation, wherein an oxidizing agent and a reducing agent are added to the pre-extracted lignocellulosic materials to compensate for the extracted organics; and c) a digester, wherein the lignocellulosic materials are separated into fibers to produce pulp.
 33. A method for producing pulp comprising the steps of: a) extracting hemicellulosic materials from lignocellulosic materials; b) treating the lignocellulosic materials with an oxidizing agent by a process selected from the group consisting of an extraction process, an agent impregnation process, and a first pretreatment process; c) treating the lignocellulosic materials with a reducing agent by a process selected from the group consisting of the extraction process, the agent impregnation process, and a second pretreatment process; and then d) subjecting the lignocellulosic materials to a modified Kraft pulping process to produce pulp.
 34. The method as claimed in claim 33, wherein the lignocellulosic materials are treated at a temperature between about 120° C. and 180° C. in the extraction process.
 35. The method as claimed in claim 34, wherein the oxidizing agent is selected from the group consisting of polysulfides.
 36. The method as claimed in claim 35, wherein the reducing agent is selected from the group consisting of lithium aluminum hydride, alkali metal borohydride, alkali metal dithionite(hydrosulfite), alkali metal amalgam, diisobutylaluminum hydride, oxalic acid, formic acid, and/or the presence of hydrogen gas in a hydroxyl ion containing solution such as Kraft white liquor.
 37. The method as claimed in claim 36, wherein the method comprises a double extraction process with water in a first extraction stage and with the oxidizing agent along with an additive in a second extraction stage or in the first pretreatment process, followed by the modified Kraft pulping process with anthraquinone in a digester.
 38. The method as claimed in claim 36, wherein the method comprises a single stage extraction process, followed by a two stage modified Kraft pulping comprising a pretreatment with the oxidizing agent.
 39. The method as claimed in claim 36, wherein the method is a single stage oxidizing agent pretreatment process for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with a polysulfide recycle.
 40. The method as claimed in claim 39, further comprising the steps of: a) impregnating hemicellulose extracted lignocellulosic materials with the oxidizing agent, wherein the oxidizing agent is an aqueous polysulfide ion containing solution, whereby aldehyde end groups of the hemicellulose extracted lignocellulosic materials are oxidized to carboxyl groups; b) treating the impregnated lignocellulosic materials with a reducing agent, whereby the aldehyde end groups of the hemicellulose extracted lignocellulosic materials are reduced to alditols or thioalditols, and subsequently continuously digesting the lignocellulosic materials to allow the lignocellulosic materials to separate into fibers without much mechanical action.
 41. The method as claimed in claim 40, further comprising adding additional additives selected from the group consisting of anthraquinone, anthraquinone derivatives, and reducing agents.
 42. The method as claimed in claim 41, wherein a portion of the aqueous extract from the extracting process is recycled back to the extraction process for raising the sugar content of the hemicellulosic materials.
 43. The method as claimed in claim 36, wherein the method is a single stage dual pretreatment with an oxidizing agent and a reducing agent for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials without a polysulfide recycle.
 44. The method as claimed in claim 43, further comprising the steps of: a) impregnating hemicellulose extracted lignocellulosic materials with the oxidizing agent, wherein the oxidizing agent is an aqueous polysulfide ion containing solution, whereby aldehyde end groups of the hemicellulose extracted lignocellulosic materials are oxidized to carboxyl groups; b) treating the impregnated lignocellulosic materials with a reducing agent, whereby the aldehyde end groups of the hemicellulose extracted lignocellulosic materials are reduced to alditols or thioalditols, and subsequently continuously digesting the lignocellulosic materials to allow the lignocellulosic materials to separate into fibers without much mechanical action.
 45. The method as claimed in claim 44, further comprising adding additional additives selected from the group consisting of anthraquinone, anthraquinone derivatives, and reducing agents.
 46. The method as claimed in claim 45, wherein a portion of the aqueous extract from the extraction process is recycled back to the extraction process for raising the sugar content of the hemicellulosic materials.
 47. The method as claimed in claim 36, wherein the process is a two stage pretreatment process with an oxidizing agent followed by a reducing agent for alkaline pulping of hemicellulose pre-extracted lignocellulosic materials with a polysulfide recycle.
 48. The method as claimed in claim 47, further comprising the steps of: a) impregnating hemicellulose extracted lignocellulosic materials with the oxidizing agent, wherein the oxidizing agent is an aqueous polysulfide ion containing solution, whereby aldehyde end groups of the hemicellulose extracted lignocellulosic materials are oxidized to carboxyl groups; b) treating the impregnated lignocellulosic materials with a reducing agent, whereby the aldehyde end groups of the hemicellulose extracted lignocellulosic materials are reduced to alditols or thioalditols, and subsequently continuously digesting the lignocellulosic materials to allow the lignocellulosic materials to separate into fibers without much mechanical action.
 49. The method as claimed in claim 48, further comprising adding additional additives selected from the group consisting of anthraquinone, anthraquinone derivatives, and reducing agents.
 50. The method as claimed in claim 49, wherein a portion of the aqueous extract from the extraction process is recycled back to the extraction process for raising the sugar content of the hemicellulosic materials. 